1. Field of the Invention
The invention relates to a method for selectively removing Zn from an aqueous solution, for example a filtrate originating from a pressure leaching process for leaching Zn and/or Pb out of a powdered material, such as blast furnace dust, containing iron.
2. Description of the Prior Art
A leaching process useful in the present method is known for example from EP-A-244910. That application describes how blast furnace dust is first separated by means of hydrocycloning into a low content Zn and Pb fraction which is suitable to be re-used in the blast furnace process, and a high content Zn and Pb fraction which undergoes further treatment.
More specifically, EP-A-244910 describes a process in which an aqueous slurry of blast furnace dust, comprising Zn, Pb and Fe is subjected to mechanical agitation, under an O.sub.2 atmosphere under pressure and at elevated temperature. A HCl-containing reagent such as waste pickling liquor (FeCl.sub.2 /HCl) is added and the pH adjusted to from 2.6 to 3. The mixture is then filtered while hot to remove a solid material containing Fe and leaving a filtrate rich in Zn and Pb. On cooling, PbCl.sub.2 crystallises out of the filtrate whilst the Zn-containing compounds remain in solution.
The above document suggests that the zinc is recovered from the remaining solution by treatment with sodium hydroxide, although no conditions of pH or temperature are recommended. The zinc content in such a precipitate may be up to approximately 30 wt %. However, the relatively Zn-rich fraction obtained in this way is not suitable for use as raw material for the zinc industry because high concentrations of Cl ions are present in the deposit. In this case, high concentrations of Cl are considered to be percentage weights of over 1%.
A further drawback with the aforementioned process is that the Zn content of the deposit obtained in this way is still relatively low for re-use in the Zn industry. For optimum processing quality of the solid matter obtained, it is desirable for the Zn content to be approximately 70%.
Zn may also be selectively removed from blast furnace dust by a leaching process which employs H.sub.2 SO.sub.4. JP Kokai No 53-4705 describes a process in which H.sub.2 SO.sub.4, H.sub.2 O.sub.2 and slaked lime are used to separately recover Zn and Fe. Fe(OH).sub.3 is precipitated at pH5 and Zn(OH).sub.2 at pH 7-8. JP Kokai No 52-82616 relates to a process for separating Fe, Zn, Pb and C in secondary blast furnace dust by treatment with H.sub.2 SO.sub.4. In the final stage Zn(OH).sub.2 is recovered by the addition of slaked lime at pH 6.5 to 9.
The problems associated with high chloride ion concentration in the final Zn precipitate clearly do not apply to these processes which employ H.sub.2 SO.sub.4 as a leaching agent.
However, as HCl-containing reagents, such as pickling liquor, are more readily available, the present invention seeks to provide a process for selectively removing Zn from a filtrate originating from leaching using HCl-containing reagent, while giving Zn deposits having lower concentrations of Cl ions, compared to processes currently available.
3. Summary of the Invention
According to the invention there is provided a method for selectively separating Zn from an aqueous solution containing ions of Zn and Fe which method comprises
(i) providing a solution containing ions of Zn and Fe obtained by leaching of blast furnace dust using hydrochloric acid, PA1 (ii) adjusting the pH of the solution to from 4.0 to 4.3 to precipitate Fe and removing the precipitate, PA1 (iii) adjusting the pH of the resultant solution with alkali to precipitate Zn.
Therefore, in conjunction with EP-A-244910, the present invention gives a method by which dust originating from blast furnaces may be fully processed into two main flows of re-useable solid raw materials, one for the blast furnace process itself, the other for the zinc and lead production industries.
A particular advantage of the method in accordance with the invention is that the liquid remaining following the second precipitation stage can largely meet environmental control requirements directed at waste flows discharged directly into the sea.
The present process is particularly suitable for treating liquids containing at least 10,000 ppm of chloride ions, obtained by leaching blast furnace dust using HCl.
The method in accordance with the invention also has the advantage that in the first stage of the Zn precipitation, undesirable elements such as Si, Al and Fe are largely deposited out of the solution, and may, therefore, be removed. In the subsequent second stage of the precipitation, highly effective precipitation of essentially zinc may then take place. Furthermore, most of the Pb is removed from the solution in the second stage. The present inventors have discovered that efficient control of pH during the first stage of this precipitation is critical, as may be seen from FIGS. 1 and 2 of the accompanying drawings. If the pH is too low, that is just below 4, then too little of the undesirable elements will be deposited. If on the other hand the pH is too high, that is just above 4.3, then besides the said undesirable elements precipitating, too much zinc will precipitate and moreover the precipitate will not be easy to filter. The pH may be adjusted by addition of an alkali, such as an alkali metal hydroxide or an alkaline earth metal hydroxide, for example.
Control of the pH during the second stage of the precipitation is not so critical. However, the pH may preferably be adjusted to a value of between 8 and 9.2 to obtain a precipitate having a reduced Cl ion concentration.
It is also desirable that precipitation in the first stage is carried out at a temperature of at least approximately 60.degree. C. Under these conditions, the filtering quality of the slurry and the settling speed of the resultant precipitate are improved. In addition the second precipitation stage is preferably carried out at a temperature of at least approximately 45.degree. C. This results in an improved quality of precipitate in which there is less absorption of chlorine. This is thought to be due to the precipitate containing a reduced amount of zinc oxychloride in comparison to those obtained in the prior art. Naturally this improves the processing quality of the solid matter deposited as raw material for the zinc industry.
The effect of temperature and pH variation on the weight percent of chloride in the filter cake obtained in a one step precipitation process is indicated in FIGS. 3 and 4 of the accompanying drawings.